int pwmPin = 6;

void setup() {
  pinMode(pwmPin, OUTPUT);
}

void loop() {
  for(int value=0; value<=255; value++) {
    analogWrite(pwmPin, value);
    delay(5);
  }
  delay(5);

  for(int value=255; value>=0; value--) {
    analogWrite(pwmPin, value);
    delay(5);
  }
  delay(5);
}